1. Field of the Invention
The present invention relates to image input devices, and more particularly, to an image input device such as a tablet device incorporating a combined display and input used for a personal computer, a word processor, and the like.
2. Description of the Background Art
As a means for inputting a written character and a diagram in a computer, a word processor, and the like, a tablet device incorporating a combined display and input is disclosed in Japanese Patent Laying-Open No. 62-135966. This tablet device is formed of a liquid crystal display and an electrostatic induction type tablet stacked on each other, for example. This tablet device enables input of a character and a diagram in the electrostatic induction type tablet as if the character and the diagram were written on a piece of paper with a writing tool. However, in the tablet device incorporating a combined display and input, a grid-like electrode pattern is observed on a display screen due to differences in reflectance and transmittance of light between a portion including electrodes and a portion not including electrodes in the tablet, thereby degrading the quality of the liquid crystal display.
As a tablet overcoming such a shortcoming, a tablet device incorporating a combined display and input shown in FIG. 19 is disclosed in Japanese Patent Laying-Open No. 5-53726.
This tablet device has an electrode serving both as a display electrode of the liquid crystal display and as a coordinates detection electrode of the electrostatic induction type tablet. As shown in FIG. 20, a coordinates detection period for detecting indicated coordinates on the tablet and a display period for displaying an image are provided in a processing time for one frame, to carry out coordinates detection and image display in a time division manner. The coordinates detection period includes an x-coordinate detection period for detecting an x value of the coordinates and a y-coordinate detection period for detecting a y value of the coordinates.
Referring to FIG. 19, the tablet device incorporating a combined display and input includes a liquid crystal panel 1 having liquid crystal sealed between common electrodes and segment electrodes arranged orthogonally, a common driving circuit 2 for driving the common electrodes, a segment driving circuit 3 for driving the segment electrodes, a switching circuit 4 for carrying out switching between display control and detection control, a display control circuit 5 for carrying out display control of an image, a detection control circuit 6 for detecting indicated coordinates, a control circuit 7 for controlling display control circuit 5, detection control circuit 6, and the like, a detection pen 8 for inputting coordinates data by indicating an arbitrary point on liquid crystal panel 1, an amplifier 9 for amplifying a signal from detection pen 8, an x-coordinate detecting circuit 10 for detecting the x value of the coordinates based on the signal amplified by amplifier 9, a y-coordinate detecting circuit 11 for detecting the y value of the coordinates based on the signal amplified by amplifier 9, and a d.c. power supply circuit 12 for supplying current to the circuits.
Liquid crystal panel 1 is formed of common electrodes Y.sub.1 to Y.sub.n (in the following description, an arbitrary common electrode is indicated as Y) and segment electrodes X.sub.1 to X.sub.m (in the following description, an arbitrary segment electrode is indicated as X) arranged orthogonally to each other with liquid crystal sealed therebetween. Each pixel is formed in a region where each common electrode Y and each segment electrode X cross each other. More specifically, liquid crystal panel 1 includes pixels of n.times.m dots arranged in a matrix manner.
This tablet device incorporating a combined display and input has an advantage of suppressing appearance of the grid-like electrode pattern on the display screen to offer a more comfortable liquid crystal display as compared to the device formed by stacking the electrostatic induction type tablet on the liquid crystal display. Further, since the liquid crystal display and the electrostatic induction type tablet can share many electrodes and driving circuits, the tablet device has an advantage of easily reducing cost, and size and weight.
The tablet device incorporating display and input combined shown in FIG. 19 operates as follows. Common driving circuit 2 for driving common electrode Y and segment driving circuit 3 for driving segment electrode X are connected to display control circuit 5 and detection control circuit 6 through switching circuit 4. Switching circuit 4 is controlled by control circuit 7, and provides an output signal from display control circuit 5 to common driving circuit 2 and segment driving circuit 3 in the display period. On the other hand, switching circuit 4 provides an output signal from detection control circuit 6 to common driving circuit 2 and segment driving circuit 3 in the coordinates detection period.
In the display period, shift data s, an inverting signal fr, a clock signal cp1, a clock signal cp2, and display data D0 to D3 are output from a shift data output terminal S, an inverting signal output terminal FR, a clock output terminal CP1, a clock output terminal CP2, and data output terminals D0 to D3 of display control circuit 5, respectively.
Clock signal cp1 whose cycle is a period for displaying pixels for one line is applied to a clock signal input terminal YCK of common driving circuit 2 and a latch pulse input terminal XLP of segment driving circuit 3 through an output terminal CP1O of switching circuit 4 as a clock signal cp1o. Shift data s, which is a pulse signal for selecting a particular common electrode Y, is applied to a shift data input terminal DIO1 of common driving circuit 2 through an output terminal SO of switching circuit 4 as shift data so in synchronism with clock signal cp1o.
When shift data so is applied to common driving circuit 2, the pulse position of shift data so is shifted by shift registers in synchronism with clock signal cp1o. A driving pulse of a common electrode drive signal is applied to common electrodes Y.sub.1 to Y.sub.n Of liquid crystal panel 1 from output terminals O1 to On of common driving circuit 2 corresponding to the shifted position. The common electrode drive signal is generated based on bias power sources V.sub.0 to V.sub.5 supplied from d.c. power supply circuit 12.
Clock signal cp2 has a cycle corresponding to a period obtained by dividing the period for displaying pixels for one line into plural periods. Clock signal cp2 is applied to a clock input terminal XCK of segment driving circuit 3 through an output terminal CP2O of switching circuit 4 as a clock signal cp2o.
Display data D.sub.0 to D.sub.3 are applied to input terminals XDO to XD3 of segment driving circuit 3 through output terminals D0O to D3O of switching circuit 4 as display data D.sub.0 o to D.sub.3 o, and sequentially input in registers in segment driving circuit 3 in synchronism with clock signal cp2o. When display data corresponding to pixels for one line are all input, the input display data are latched at a timing of clock signal cp1o which is applied to latch pulse input terminal XLP, and the driving pulse of the segment electrode drive signal corresponding to each display data is applied to segment electrodes X.sub.1 to X.sub.m of liquid crystal panel 1 from output terminals O1 to Om of segment driving circuit 3. This segment electrode drive signal is also generated based on bias power sources V.sub.0 to V.sub.5 supplied from d.c. power supply circuit 12.
Note that inverting signal fr prevents degradation of a liquid crystal due to electrolysis by periodically inverting the direction of voltage application to the liquid crystal in the display period. Inverting signal fr is applied to an inverting signal input terminal YFR of common driving circuit 2 and an inverting signal input terminal XFR of segment driving circuit 3 through an inverting signal output terminal FRO of switching circuit 4 as an inverting signal fro.
The pixel matrix of liquid crystal panel 1 is driven by the operation of common driving circuit 2 and segment driving circuit 3 according to the line order, and an image corresponding to display data D.sub.0 to D.sub.3 is displayed on liquid crystal panel 1.
On the other hand, during the coordinates detection period, shift data sd, an inverting signal frd, a clock signal cp1d, a clock signal cp2d, and drive data D.sub.0 d to D.sub.3 d are output from a shift data output terminal Sd, an inverting signal output terminal FRd, a clock output terminal CP1d, a clock output terminal CP2d, and data output terminals D0d to D3d of detection control circuit 6, respectively.
Clock signal cp1 has a cycle corresponding to a scanning period for scanning one common electrode Y. Clock signal cp1 is applied to clock input terminal YCK of common driving circuit 2 and latch pulse input terminal XLP of segment driving circuit 3 through output terminal CP1O of switching circuit 4 as clock signal cp1o. Shift data sd which is a pulse signal for selecting a particular common electrode Y is applied to shift data input terminal DIO1 of common driving circuit 2 through output terminal SO of switching circuit 4 as shift data so in synchronism with clock signal cp1d.
Similar to the case of the above described display period, the pulse position of shift data so is shifted by shift registers included in common driving circuit 2 in synchronism with clock signal cp1o, and scanning pulses of common electrode scan signals y.sub.1 to Y.sub.n (in the following description, an arbitrary common electrode scan signal is indicated as y) are sequentially applied to common electrodes Y.sub.1 to Y.sub.n from output terminals O1 to On corresponding to the shifted position. This common electrode scan signal y is generated based on bias power sources V.sub.0 to V.sub.5 supplied from d.c. power supply circuit 12.
Clock signal cp2d has a cycle corresponding to a scanning period for scanning segment electrode X. Clock signal cp2d is applied to clock input terminal XCK of segment driving circuit 3 through output terminal CP2O of switching circuit 4 as clock signal cp2o.
Drive data D.sub.0 d to D.sub.3 d are applied to input terminals XD0 to XD3 of segment driving circuit 3 through output terminals D0O to D3O of switching circuit 4 as drive data D.sub.0 o to D.sub.3 o, and sequentially input in registers in segment driving circuit 3 in synchronism with clock signal cp2o. Scanning pulses of segment electrode scan signals x.sub.1 to x.sub.m (in the following description, an arbitrary segment electrode scan signal is indicated as x) corresponding to the above described drive data are output from output terminals O1 to Om of segment driving circuit 3 to segment electrodes X.sub.1 to X.sub.m. This segment electrode scan signal x is also generated based on bias power sources V.sub.0 to V.sub.5 supplied from d.c. power supply circuit 12.
FIG. 21 is a timing chart of the respective scan signals in the coordinates detection period of the tablet device incorporating the combined display and input device of FIG. 19.
The coordinates detection period is divided into the x-coordinate detection period and the y-coordinate detection period following thereto. During the x-coordinate detection period, segment electrode scan signal x which is a pulse voltage signal is sequentially applied to segment electrode X. On the other hand, in the y-coordinate detection period, common electrode scan signal y, which is a pulse voltage signal, is sequentially applied to common electrode Y.
By application of the pulse voltage signal, a voltage is induced in detection pen 8 by stray capacitance between segment electrode X or common electrode Y and a tip electrode of detection pen (indicated coordinates detection pen) 8. The induced voltage generated in detection pen 8 is amplified by amplifier 9, and then applied to x-coordinate detecting circuit 10 and y-coordinate detecting circuit 11.
Based on an output signal from amplifier 9 and a timing signal from control circuit 7, x-coordinate detecting circuit 10 and y-coordinate detecting circuit 11 detect the x value of the coordinates or the y value of the coordinates of the position indicated by detection pen 8 by detection of a time period from application of the pulse voltage signal to the induced voltage attaining the maximum value.
However, the above described tablet device incorporating a combined display and input has the following problems.
FIG. 22 shows the problems in the conventional tablet device incorporating a combined display and input.
In the graph of FIG. 22, the abscissa represents an actual x value of the coordinates indicated by the detection pen, and the ordinate represents a detected x value of the coordinates.
It is desired that the indicated x value of the coordinates and the detected x value of the coordinates have a linear relationship as shown by the dotted line. However, in portions proximate to upper and lower ends of the liquid crystal panel, the indicated x value of the coordinates and the detected x value of the coordinates have a relationship of a curve having a periodic vibration as shown by the solid line. The amplitude of the vibration becomes larger as the detected coordinates approaches the upper and lower ends of the liquid crystal panel.
More specifically, in the portions proximate to the upper and lower ends of the screen, an accurate detected coordinates value cannot be obtained.
FIG. 26 shows display when straight line images are input at an equal interval in the longitudinal direction by the detection pen in the conventional tablet device incorporating a combined display and input.
As shown in the figure, even if a diagram actually input is a straight line, a periodic error is produced in the detected x value of the coordinates in the vicinity of an end portion 100 on the screen as described above. The error becomes larger as the straight line approaches end portion 100 of the screen. Positions P1 to P5 of the x value of the coordinates in which the error is produced in FIG. 26 corresponded to positions P1 to P5 of FIG. 22, respectively.
The error is produced by the following reason. As shown in FIG. 23, scan electrodes (in this case, segment electrodes) included in the liquid crystal panel are arranged in parallel in a screen 102. However, the scan electrodes converge on driver ICs (integrated circuits) 104a and 104b outside screen 102. Therefore, the electrodes are distributed more closely in the vicinity of IC outside screen 102.
When there is no variation in distribution of the electrodes as shown in FIG. 24A (for example, the center portion of the liquid crystal panel), and a voltage is sequentially applied to the scan electrodes in the scanning direction, the voltage induced in the detection pen becomes larger as the electrode to which the voltage is applied approaches the position indicated by the detection pen. The voltage induced in the detection pen attains the maximum value when the voltage is applied to an electrode indicated by the detection pen. As the electrode to which the voltage is applied becomes remote from the indicated position of the detection pen, the induced voltage decreases.
As described above, when there is no variation in the distribution of the electrodes, the maximum point of the induced voltage and the position indicated by the detection pen match.
However, in the vicinity of a portion (portion indicated by "A") where there is a variation in the distribution of the scan electrodes as shown in FIG. 25A, the detection pen is influenced by the voltage applied to the portion "A". Therefore, as shown in FIG. 25B, the position indicated by the detection pen and the maximum point of the induced voltage do not match. The maximum point of the induced voltage is shifted to the side of the portion "A" where the electrodes are less closely arranged (the left side in the figure). The more the position indicated by the detection pen approaches the portion "A", the larger the detection error becomes.
In order to correct such a detection error as described above, a method is considered for correcting the detected coordinates using a stored correction value corresponding to each pixel in the portions proximate to the upper and lower ends of the screen. However, it takes time to carry out correction with this method. A quick processing of a signal as in the tablet device reading a coordinates value by a digital signal disclosed in Japanese Patent Laying-Open 62-135966 cannot be implemented with this method.